Weird Stellar Pair Puzzles Scientists

Astronomers have discovered a speedy spinning pulsar in an elongated
orbit around an apparent Sun-like star, a combination never seen before,
and one that has them puzzled about how the strange system developed.

Comparing Orbits of Pulsar and Its Companion to our Solar System.CREDIT:Bill Saxton, NRAO/AUI/NSFClick on image for full caption information and available
graphics.

"Our ideas about how the fastest-spinning pulsars are produced do
not predict either the kind of orbit or the type of companion star
this one has," said David Champion of the Australia Telescope
National Facility. "We have to come up with some new scenarios
to explain this weird pair," he added.

Astronomers first detected the pulsar, called J1903+0327, as part of a
long-term survey using the National Science Foundation's Arecibo
radio telescope in Puerto Rico. They made the discovery in 2006
doing data analysis at McGill University, where Champion worked
at the time. They followed up the discovery with detailed studies
using the Arecibo telescope, the NSF's
Robert C. Byrd Green Bank
Telescope (GBT) in West Virginia, the Westerbork radio telescope
in the Netherlands, and the Gemini North optical telescope in Hawaii.

The
pulsar, a city-sized superdense stellar corpse left over after
a massive star exploded as a
supernova, is spinning on its axis
465 times every second. Nearly 21,000 light-years from Earth, it
is in a highly-elongated orbit that takes it around its companion
star once every 95 days. An infrared image made with the Gemini North
telescope in Hawaii shows a Sun-like star at the pulsar's position.
If this is an orbital companion to the pulsar, it is unlike any
companions of other rapidly rotating pulsars. The pulsar, a
neutron star, also is unusually massive for its type.

"This combination of properties is unprecedented. Not only does it
require us to figure out how this system was produced, but the
large mass may help us understand how matter behaves at extremely
high densities," said Scott Ransom of the National Radio
Astronomy Observatory.

Pulsars are
neutron stars whose strong magnetic fields channel
lighthouse-like beams of light and radio waves that whirl
around as the star spins. Typical pulsars spin a few times a
second, but some, like PSR J1903+0327, are much faster, rotating
hundreds of times a second. They are called
millisecond pulsars.

Astronomers think most millisecond pulsars are sped up by material
falling onto them from a companion star. This requires the pulsar
to be in a tight orbit around its companion that becomes more and
more circular with time. The orbits of some millisecond pulsars
are the most perfect circles in the Universe, so the elongated
orbit of the new pulsar is a mystery.

"What we have found is a millisecond pulsar that is in the
wrong kind of orbit around what appears to be the wrong kind of
star," Champion said. "Now we have to figure out how this strange
system was produced."

The scientists are considering three possibilities. The first,
that the pulsar simply was born spinning quickly, seems
unlikely to them. Another possibility, they say, is that the
pulsar was formed in a tight group of stars known as a
globular
cluster, where it had a companion that spun it up. Later, a
close encounter with another star in the cluster stripped it
of its companion and flung it out of the cluster. For several
reasons, including the fact that they don't see a nearby
cluster from which it could have come, they don't like that
explanation either.

A third scenario says the pulsar may be part of a triple,
not a double, star system. In this case, the pulsar's 95-day
orbit is around a neutron star or
white dwarf, not the
Sun-like star seen in the infrared image. The Sun-like star
would then be in a more-distant orbit around the pulsar
and its close companion.

"We've found about 50 pulsars in binary systems. We may now
have found our first pulsar in a stellar triple system,"
Ransom said.

The international research team is busy trying to get their
answers. They will study the star in the infrared image further
to confirm the indications that it is similar to our Sun and
that it actually is a companion to the pulsar. Additional radio
observations will study the pulsar's orbit and seek to measure
its motion in space.

"This is a fascinating object that has a lot to teach us about
physics. It's going to be exciting to peel away the mystery
of how this thing came to be," Champion said.

Meanwhile the survey with Arecibo goes on. Other, more conventional
millisecond pulsars have been found, but it is almost certain that
there will be more surprises, said Jim Cordes of Cornell University,
chair of the consortium that is doing the survey. The large volume
of data for the survey is archived at the Cornell Center for Advanced
Computing, and is then processed using computer clusters at several
consortium-member sites around the world.

The National Astronomy and Ionosphere Center is operated by Cornell
University under a cooperative agreement with the National Science
Foundation.